Fiber pump combiner

ABSTRACT

An optical fiber combiner comprising a coupling device having an input surface area, and you A in , and an output surface area, A out , wherein the input surface area A in  is greater than the output surface area A out , and a plurality of optical fibers each having an input surface and an output surface, wherein the output surfaces of the plurality of optical fibers are coupled to the coupling device, wherein the coupling device combines optical power emitted by the plurality of optical fibers.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/391,050 filed Dec. 27, 2016, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The disclosure relates to high power fiber laser systems. Particularly,the disclosure relates to a high power fiber system configured tocombine a plurality of optical fibers into a single output fiber.

BACKGROUND

High power semiconductor lasers are used to pump cladding pumped fiberlasers. Fiber lasers are capable of producing output power in themultiple kW range and are used in a variety of applications that requirehigh output power such as cutting, welding, material processing (e.g.,marking, engraving, and cutting) and directed energy. Achieving thepower levels required for these applications is often accomplished bycombining the fiber-coupled outputs of multiple lower power diodemodules to pump active fibers.

When combining diode pumped fibers it is often convenient to perform thebeam combination of the coupled fibers with a fiber based beam combinerthat couples a plurality of optical fibers to a single output fiber.Conventionally, combining multiple fibers to achieve higher power caneither reduce beam efficiency or beam quality. High power(kilowatt-class) fiber pump, pump-signal and signal combiners arevulnerable to small imperfections and losses which have significantimpact on reliability.

In general, a process of fabricating a high power combiner includessignificant physical manipulation of the optical fibers in the bundle.For example, often optical output fibers of the bundle are fused,twisted and tapered into an hourglass shape, and cleaved at the waist.Conventionally, tapering of the bundle of fibers requires thecross-sectional area of the untapered fiber bundle to be narrowed downto a cross-sectional area of the single output fiber. Once the fiberbundle is cleaved, the output end of the tapered fiber bundle is spliced(or otherwise coupled) onto an output fiber. All of this physicalmanipulation may result in structural defects such as micro-bends in thefibers or cladding which can introduce loss and/or degradation in beamquality or efficiency. Further, due to the twisting of optical fibersneeded to achieve high coupling efficiency they are difficult to handle.What is needed is a method of reducing the amount of physicalmanipulation of the optical fibers associated with the fabrication ofoptical fiber combiners.

SUMMARY

Disclosed herein is an optical fiber combiner comprising, a couplingdevice having an input surface area, A_(in), and an output surface area,A_(out), wherein the input surface area A_(in) is greater than theoutput surface area A_(out) and wherein a body of the coupling devicecomprises a gradual taper from the input surface area to the outputsurface area, and a plurality of optical fibers each having an inputsurface and an output surface, wherein the output surfaces of theplurality of optical fibers are optically coupled to the couplingdevice, wherein the coupling device combines optical power emitted bythe plurality of optical fibers. The optical fiber combiner may alsoinclude a hollow structure configured to encase the plurality of opticalfibers and to secure the plurality of optical fibers in place forcoupling to the coupling device. In some examples, the hollow structureis a capillary having an inner diameter substantially equal to an outerdiameter of the input surface area of the coupling device. In someexamples, wherein the hollow structure is closely-fit around the outputsurface area of the coupling device and the optical fibers. The outputsurfaces of the plurality of optical fibers may be substantially incontact with the input surface area of the coupling device. There may bea gap between the output surfaces of the plurality of optical fibers andthe input surface area of the coupling device, wherein the plurality ofoptical fibers are held in place by the hollow structure. The hollowstructure may have a shape substantially matching the shape of the inputsurface of the coupling device, wherein the shape is circular,elliptical, rectangular, polyhedral, or any combinations thereof. Thehollow structure may comprise a material configured to constrict aroundthe plurality of optical fibers and the coupling device in response tomechanical means, pressure change, exposure to a chemical, or a chemicalcatalyst, or any combinations thereof.

The output ends of the optical fibers can be coated with anantireflective coating. In some examples, the hollow structure has alower index of refraction than the index of refraction of the pluralityof input optical fibers. The output end of the optical fibers can becoupled to the input surface of the coupling device by plasma heating,CO₂ laser, resistive heating, fusion splicing or epoxy or anycombinations thereof.

Further disclosed herein is a method for fabricating an optical fibercombiner comprising, exposing an output end of a silica rod to anetchant, gradually exposing a length of the silica rod lengthwise to theetchant over a period of time, ending the etching at the input end toetch a gradual taper to into the length of the silica rod by graduallyexposing the length of the silica rod to the etchant over the period oftime, wherein the etching begins by exposing the output end to theetchant first and then gradually exposing the entire length of thesilica rod ending the etching with the input end causing the input endto have a greater surface area A_(in) than the surface area A_(out) ofthe output end. The method may also include gradually exposing thesilica rod to the etchant at a constant rate for a fixed period of time.In some examples, the fixed period of time is determined by an outputdiameter of the silica rod. The method may also include polishing theinput end of the silica rod. The etching can be wet etching. The methodmay further include, coating the input surface with an antireflectivecoating for free-space coupling and/or coating the input surface with asurface treatment configured to enable fusion splicing of input fibers.

Further disclosed herein is a method for fabricating an optical fibercombiner, comprising, applying heat to a silica rod to fabricate agradual taper in the silica rod over a length of the silica rod, whereinthe silica rod has an input end and an output end, wherein the silicarod is tapered such that the input end has a greater surface area A_(in)than a surface area A_(out) of the output end, treating the input end ofthe silica rod with an anti-reflective coating and forming an outputwaveguide onto the output end of the silica rod. In an example, theoutput waveguide further comprises splicing the waveguide onto theoutput end of the silica rod.

Also disclosed herein is a method for fabricating an optical fibercombiner comprising, disposing a first end of a hollow structure aroundan input surface of a coupler, the coupler comprising a silica rodhaving a tapered outer surface and an output waveguide, wherein thecoupler has a higher index of refraction compared to the hollowstructure, threading a plurality of optical fibers from a second end ofthe hollow structure through a length of an inner aperture of the hollowstructure, coupling the plurality of optical fibers with the inputsurface of the coupler and collapsing the hollow structure onto thecoupler and the plurality of optical fibers. In an example, an outersurface of the hollow structure is textured to remove high numericalaperture light transmitted into the optical fibers or the hollowstructure or a combination thereof. The hollow structure may have ahigher index of refraction compared to an index of refraction of inputfiber cladding. In an example, the hollow structure may have an index ofrefraction that only strips light having a predetermined numericalaperture that is not desirable to be coupled into the coupler. Thehollow structure may have an asymmetric shape and is configured toscramble the mode.

The foregoing and other objects, features, and advantages will becomemore apparent from the following detailed description, which proceedswith reference to the accompanying figures which may not be drawn toscale.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, wherein like reference numerals representlike elements, are incorporated in and constitute a part of thisspecification and, together with the description, explain the advantagesand principles of the presently disclosed technology. In the drawings,

FIG. 1 illustrates an example of an optical fiber combiner assembly;

FIG. 2 illustrates an example of a hollow structure configured to securea bundle of one or more optical fibers in place;

FIG. 3 is a cutaway view illustrating an example of an optical fibercombiner assembly;

FIG. 4A is a cutaway view illustrating an example of an optical fibercombiner assembly;

FIG. 4B is a cross-sectional view illustrating an example of a hollowstructure disposed around coupler;

FIG. 4C is a cross-sectional view illustrating an example of a hollowstructure disposed around a bundle of optical fibers;

FIG. 4D is a cutaway view illustrating an example of an optical fibercombiner during assembly;

FIG. 5A is a cutaway view illustrating an example of an optical fibercombiner assembly;

FIG. 5B is a cross-sectional view of a bundle of optical fibers.

FIG. 5C is a cross-sectional view of a bundle of optical fibers having acoating on outward facing surfaces of the optical fibers;

FIG. 6A is a cutaway view illustrating an example of an optical fibercombiner assembly;

FIG. 6B is a sectional view of an example of a bundle of optical fiberswithin a hollow structure having retained cladding;

FIG. 7 is a cutaway view illustrating an example of an optical fibercombiner assembly;

FIG. 8 is a cutaway view illustrating an example of an optical fibercombiner assembly;

FIG. 9 illustrates an example process for fabricating an optical fibercombiner;

FIG. 10 illustrates an example process for fabricating an optical fibercombiner;

FIG. 11 illustrates an example process for fabricating an optical fibercombiner;

FIGS. 12A-12D depict examples of hollow structures having differingsymmetries; and

FIG. 12E depicts graphs of beam-parameter product (BPP) conversionefficiency for various hollow structure symmetries.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

In some examples, values, procedures, or apparatus' are referred to as“lowest”, “best”, “minimum,” or the like. It will be appreciated thatsuch descriptions are intended to indicate that a selection among manyused functional alternatives can be made, and such selections need notbe better, smaller, or otherwise preferable to other selections.Examples are described with reference to directions indicated as“above,” “below,” “upper,” “lower,” and the like. These terms are usedfor convenient description, but do not imply any particular spatialorientation.

FIG. 1 illustrates an example of an optical fiber combiner assembly 100.In an example, optical fiber combiner assembly 100 comprises a coupler102 that is configured to taper to an output waveguide 104. The coupler102 couples to waveguide 104 at an output surface 120. An input surface106 of coupler 102 is positioned to receive optical energy emitted fromone or more input optical fibers 108. Input surface area of coupler 102has an input surface area of A_(in) and a numerical aperture NA_(in).Output surface 120 has an output surface area A_(out) and a numericalaperture NA_(out). In an example, input surface area A_(in) is greaterthan the output surface area A_(out).

Assembly 100 can form a part of a laser system wherein optical energy iscoupled into optical fibers 108 by diode pumps and/or other sources.Optical fibers 108 form a bundle 112. Optical fibers 108 can be any sizeor shape as long as the individual optical fibers 108 or the bundle 112of optical fibers 108 does not violate the brightness conservation rulefor the sum of the input fibers, (i.e.,

$\left. {\left( \frac{{NA}_{in}}{{NA}_{out}} \right)^{2} \leq \frac{A_{out}}{\sum A_{in}}} \right)$where A_(in) and NA_(in) are input fiber cross-sectional area and thenumerical aperture of the beam respectively and A_(out) and NA_(out) areoutput cross-sectional area of the coupler and its numerical aperturerespectively. Coupler 102 combines power from bundle 112 of N-inputoptical fibers 108 into output waveguide 104. Bundle 112 may compriseany number N of input optical fibers 108. The combined optical power isdirected to output surface 120 where power may be further coupled into asingle output fiber 104 for emission of optical power at an outputsurface of the output fiber. Rather than tapering the optical fibersthemselves, optical fibers 108 may be coupled to coupler 102 that isitself tapered which reduces the amount of physical manipulation of theoptical fibers 108 associated with fabrication of conventional opticalfiber combiners. The coupler 102 may be tapered from a diametersufficient for coupling optical fiber bundle 112 at the input surface106 down to a diameter at the output end 120 that is the same, similaror even smaller compared to the diameter of the input end 116 of thesingle output fiber 104.

Optical fibers 108 may be coupled to coupler 102 via a variety ofmethods including butt-coupling or by fusion. In an example, opticalfibers 108 and input surface 106 of coupler 102 may be in contact or atleast as closely in physical contact as possible. In some embodiments,gaps between optical fibers 108 and input surface 106 may arise due toimperfections in an angle of alignment between optical fibers 108 andinput surface 106. It may be desirable to minimize imperfections in theangle of alignment between optical fibers 108 and input surface 106 topreserve (i.e., not deteriorate) beam parameter product (BPP).

FIG. 2 illustrates an example of a hollow structure 210 configured tosecure bundle 112 of one or more optical fibers 108 in place in acombiner assembly 100. Hollow structure 210 may be any of a variety ofshapes, for example, a circle, rectangle, ellipse, polyhedron, anirregular shape or the like or any combinations thereof. Hollowstructure 210 may comprise sidewalls 212 and a cavity 214 into which oneor more optical fibers 108 may be disposed. Hollow structure 210 maycomprise a variety of materials including materials such as glass, fusedsilica, polymer, doped polymer, fluorosilicate, silica doped with one ormore materials known to those of skill in the art to be of judiciouschoice of index of refraction, or the like or any combination thereof.Hollow structure 210 may comprise a material configured to constrictaround the plurality of optical fibers and the coupling device inresponse to application of heat, mechanical means, pressure change orexposure to a chemical, such as a chemical catalyst.

In an example where input fibers 108 retain cladding in the bundle 112,the hollow structure 210 may be of any of a variety of shapes and highindex material compared to the cladding 308 (see FIG. 3) of the inputfibers 108. This will help to strip any higher NA light along 210 regionbefore coupling into coupler 102. Yet, in another example, hollowstructure 210 may be designed with a judicious choice of index ofrefraction to strip any higher NA light that cannot be coupled into thecoupler 102 and may cause detrimental effects in the combiner 102assembly. Outer surface 250 of hollow structure 210 may have any of avariety of surface textures. For example, the texture may be smooth ormay be roughened or textured in order to strip out unwanted higher NAlight.

FIG. 3 is a cutaway view illustrating an example of an optical fibercombiner assembly 300. In an example, optical fibers 108 are eachstripped of cladding 308 and buffer or protective coating. Fiber bundle112 and coupler 102 may be positioned within hollow structure 210.Hollow structure 210 may be a round capillary. An inner diameter ofhollow structure 210 may be the same shape and substantially equal to orslightly larger than an outer diameter of coupler 102 such that coupler102 can be compactly disposed in cavity 214 of hollow structure 210.Similarly, the inner diameter of hollow structure 210 may be the sameshape and substantially equal to or slightly larger than an outerdiameter of bundle 112 such that coupler 102 can be securely disposed incavity 214 of hollow structure 210. Thus, hollow structure 210 maysecure optical fibers 108 and coupler 102 in place. Input surface 106may be disposed a distance L, from one or more output surfaces 110 ofone or more optical fibers 108. In some examples, optical fibers 108 maybe positioned within hollow structure 210 without stripping cladding308.

In an example, output surfaces 110 of optical fibers 108 and inputsurface 106 of coupler 102 may be separated by a gap 306. Outputsurfaces 110 of optical fibers 108 and/or input surface 106 of coupler102 may be coated with an antireflection coating to facilitate low losstransmission of optical energy across gap 306. In an example, hollowstructure 210 may have a lower index of refraction compared to an indexof refraction of one or more optical fibers 108 or higher than the indexof refraction of the cladding 308 of input fibers 108.

In an example, hollow structure 210 may be fitted over an outer surfaceof coupler 102. Such a fitting may be tight and may require a certainamount of force to mate coupler 102 within aperture 214. Likewise,bundle 112 may be threaded through aperture 214 to achieve aclose-fitting of bundle 112 within hollow structure 210.

FIG. 4A is a cutaway view illustrating an example of an optical fibercombiner assembly 400. In an example, optical fibers 108 and coupler 102may be positioned within hollow structure 210. Output surfaces 110 ofoptical fibers 108 and input surface 106 of coupler 102 may be inphysical contact. Contact between output surfaces 110 and input surface106 may be made by a variety of methods such as by fusion splicingand/or van der Waals bonding. A variety of methods may be used to fusionsplice the output surfaces 110 and input surface 106 such as using alaser, a flame, an electric arc, and/or plasma or the like or acombination thereof.

In an example, unwanted higher NA light may get coupled into 210 anddissipate from the output end. The outer surface 250 of hollow structure210 may be corrugated, textured or otherwise configured to scatter thehigher NA light coupled into hollow structure 210 along the length aswell as the end to distribute the dissipation. Hollow structure 210 maybe fabricated to fit snuggly around coupler 102 and bundle 112. Tofacilitate proper alignment between optical fibers 108 and input surface106, hollow structure 210 may be used as a self-aligning guide forcoupler 102 and/or optical fibers 108 to assist in aligning outputsurfaces 110 and input 106 surfaces. Such a guide can minimizeimperfections in an angle of alignment between optical fibers 108 andcoupler 102. Hollow structure 210 may be positioned over an outerdiameter of bundle 112 and an outer diameter of coupler 102 such thatcoupler 102 is tightly disposed within central aperture 214 of hollowstructure 210. Optical fibers 108 and coupler 102 are placed withinhollow structure 210 where the inner diameter of hollow structure 210 isonly slightly larger than the outer diameter of either or optical fiberbundle 112 or coupler 102. Mating, positioning and/or alignment ofcoupler 102 and optical fibers 108 may be facilitated with tension,heat, vacuum pressure, liquid and/or index matching fluid may be used.This technique may simplify alignment, improve yield and prevent inputoptical fibers from going astray during the splicing process especiallyfor large diameter couplers (e.g., ≥300.0 micron diameter). Hollowstructure 210 may or may not be part of a final coupler assembly 400. Inother words, hollow structure 210 may be removed after fibers 108 arealigned with surface 106.

FIGS. 4B-4D illustrate various views of a combiner 400 during an exampleassembly process wherein hollow structure 210 is collapsed aroundoptical fibers 108 and combiner 102.

FIG. 4B illustrates a cross-sectional view of hollow structure 210disposed around coupler 102. In an example, during assembly of combiner400, hollow structure 210 may be loosely disposed over coupler 102 wherean inner diameter 492 of central aperture 214 is greater than an outerdiameter 490 of coupler 102 to enable fitting of hollow structure 210around coupler 102 without damaging coupler 102 or hollow structure 210.As shown in FIG. 4C, during assembly, bundle 112 of optical fibers 108may be fed through central aperture 214 of hollow structure 210 foralignment with surface 106. Inner diameter 492 of hollow structure 210at this point in the assembly of combiner 100 is greater than thediameter 494 of bundle 112 of optical fibers 108 to facilitate threadingof optical fibers 108 through hollow structure 210. During assembly,having inner diameter 492 of hollow structure 210 greater than thediameter 494 of bundle 112 of optical fibers 108 may also facilitatealignment of surfaces 110 of optical fibers 108 with surface 106 ofcoupler 102.

FIG. 4D is a cutaway view of assembly 100 prior to collapsing hollowstructure 210 around optical fibers 108 and coupler 102. In an example,heat can be applied to hollow structure 210 to collapse it aroundcoupler 102 and bundle 112. Once surfaces 110 are aligned and/or coupledwith surface 106. FIG. 4A illustrates an assembly 400 after collapsinghollow structure 210 around optical fibers 108 and coupler 102 assembly400. As can be seen in FIG. 4A, there is no longer excess space aroundoptical fiber bundle 112 after collapsing hollow structure 210. In thisconfiguration, hollow structure 210 can provide support and protectionto optical fibers of bundle 112 and coupler 102.

FIG. 5A is a cutaway view illustrating an example of an optical fibercombiner assembly 500. Optical fibers 108 may be stripped of claddingand other coatings. Output surfaces 110 of optical fibers 108 and inputsurface 106 of coupler 102 may be in physical contact without supportinghollow structure 210. Output surfaces 110 may be coupled to inputsurface 106 by any of a variety of coupling methods such as plasmaheating, CO₂ laser annealing, resistive heating, fusion splicing or useof epoxy and/or other methods known to those skilled in the art. Bundle112 may comprise a coating 504 on outward facing surfaces 502 of opticalfibers 108. Coating 504 may be a low refractive index material includinglow index materials such as glass, fused silica, polymer, doped polymer,fluorosilicate, silica doped with one or more materials known to thoseof skill in the art to lower an index of refraction of a material, orthe like or any combination thereof to strip high NA light not desirableto be coupled into the coupler 102.

FIG. 5B is a sectional view of a bundle 112 of optical fibers 108without coating 504 on outward facing surfaces 502 of optical fibers108. FIG. 5C is a sectional view of a bundle 112 having a coating 504 onoutward facing surfaces 502 of optical fibers 108. Coating 504 may be avariety of thicknesses and may conform to the shape of the outwardfacing surfaces 502 of optical fibers 108. Coating 504 may fill in gapsbetween optical fibers and may provide support to optical fibers 108.

FIG. 6A is a cutaway view illustrating an example of an optical fibercombiner assembly 600. Fibers 108 are not stripped of an outer cladding.Rather, fibers 108 retain cladding 308 within hollow structure 210.Fibers 108 may retain cladding 308 in any of the embodiments describedor contemplated herein. In FIG. 6A, output surfaces 110 of opticalfibers 108 and input surface 106 of coupler 102 are not in physicalcontact. A gap 606 of length L may separate output surfaces 110 andinput surface 106. Hollow structure 210 provides support to coupler 102and optical fibers 108. Coupler 102 may comprise an extended inputportion 602 that is not tapered. Having an untapered extended portion602 allows stripping of higher NA light that is not desirable to becoupled into the coupler 102. A tapered portion 604 may be taperedbetween the extended input portion 602 and the output portion 610.Output surfaces 110 and/or output surface 106 may be coated with anantireflective coating. The antireflective coating may facilitate lowloss transmission of optical energy across gap 606.

FIG. 6B is a sectional view of a bundle 112 of optical fibers 108 withinhollow structure 210 having retained cladding 308.

FIG. 7 is a cutaway view illustrating an example of an optical fibercombiner assembly 700. Output surfaces 110 of optical fibers 108 andinput surface 106 of coupler 102 are in physical contact. Hollowstructure 210 provides support to coupler 102 and optical fibers 108.Coupler 102 may comprise an extended input portion 602 that is nottapered. The length of the non-tapered portion 602 may be positioned soas to not extend to an exit face 702 of aperture 214. In anotherexample, the length of the non-tapered portion 602 may be positioned soas to extend to an exit face 702 of aperture 214.

FIG. 8 is a cutaway view illustrating an example of an optical fibercombiner assembly 800. Output surfaces 110 of optical fibers 108 andinput surface 106 of coupler 102 may be in physical contact withoutsupporting hollow structure 210. Output surfaces 110 may be spliced toinput surface 106 by any of a variety of fusion methods such as plasmaheating, CO₂ laser annealing, resistive heating, or use of epoxy and/orother methods known to those skilled in the art. Coupler 102 maycomprise an extended input portion 602 that is not tapered.

FIG. 9 illustrates an example process 900 for fabricating a taperedoptical fiber coupler to be coupled to a plurality of optical fibers 108in an optical fiber combiner assembly. In an example, combiner 102 canbe fabricated from a length of silica rod. Coupler 102 formed from thesilica rod may have an input end 106 and an output end 120. In anexample, the silica rod may be wet etched by gradually exposing thesilica rod to the etchant at a constant rate for a fixed period of time,wherein the fixed period of time is determined by a desired outputdiameter of the input end 106 and output end 120 of coupler 102. Theinput end 106 is to be connected to the plurality of optical fibers 108and thus has a surface area A_(in) large enough to receive the outputends 110 of N optical fibers 108 of bundle 112. After processing, theoutput surface area A_(out) will be smaller than input surface areaA_(in) as the diameter of the silica rod decreases with the taper fromthe input surface 106 to the output surface 120. The silica rod may besubstantially circular. In another example, the silica rod may be any ofa variety of shapes such as elliptical, rectangular, star shaped, apolyhedron and/or an irregular shape, or the like or any combinationsthereof. Further, the silica rod may comprise a variety of materialsand/or may be doped with a rare earth element such as ytterbium,neodymium and/or erbium, or the like or a combination thereof.

Process 900 begins at block 902, “EXPOSE AN OUTPUT END OF A SILICA RODTO AN ETCHANT.” At block 902, fabrication of coupler 102 may begin byexposing a designated output end of the silica rod to an etchant firstas the output end of the silica rod will have the smallest diameter andthus should be exposed to the etchant for the longest period of time.Process 900 proceeds to block 904, “GRADUALLY EXPOSE THE LENGTH OF THESILICA ROD TO THE ETCHANT OVER A PERIOD OF TIME, ENDING THE ETCHING ATTHE INPUT END.” Such a gradual etch will cause the input end to have agreater surface area A_(in) than the surface area A_(out) of the outputend.

Process 900 proceeds to block 906, “TREAT INPUT AND/OR OUTPUT END OFSILICA ROD.” Input surface 106 may be treated for coupling with outputends 110 of the optical fibers 108. For example, the input surface 106may be polished and/or coated with antireflective coating for free-spacecoupling. The texture of input surface 106 may be prepared to enablefusion splicing with optical fibers 108. In another example, inputsurface 106 may be coated with a surface treatment configured to promotecoupling or fusion of the input surface 106 with the output surfaces 110of the optical fibers 108.

Process 900 proceeds to block 906, “COUPLE A PLURALITY OF OPTICAL FIBERSTO THE INPUT SURFACE AREA OF THE TAPERED SILICA ROD.” One or moreoptical fibers 108 may be coupled to input surface 106 by a variety ofmethods, as discussed previously.

FIG. 10 illustrates an example process 1000 for fabricating a taperedoptical fiber coupler 102 to be coupled to a plurality of optical fibers108 to make an optical fiber combiner assembly 100. Process 1000 maybegin at block 1002, “APPLY HEAT TO A SILICA ROD TO FABRICATE A GRADUALTAPER.” At block 1002 a gradual taper is fabricated into a length of asilica rod by applying heat to a silica rod. The silica rod has an inputend 106 and an output end 120. Heat may be applied by a variety ofmethods including using a CO₂ laser, plasma and/or resistive heating, orthe like or any combinations thereof. The silica rod can be tapered inthe applied heat using mechanical means such that input end 106 willhave a greater input surface area A_(in) than the output surface areaA_(out) of the output end 120. Process 1000 may move to block 1004,“FABRICATE A WAVEGUIDE ON THE SILICA ROD.” At block 1004, an outputwaveguide 104 is fabricated by heating and tapering an end portion ofthe silica rod. In another example, a waveguide 104 portion may befabricated separately and coupled onto the end of the tapered silica rod(i.e., coupler 102). One can also first make the tapered coupler pieceto which the input fibers are fused onto input surface and waveguide 104fused to the output surface.

Process 1000 proceeds to block 1006, “TREAT INPUT AND/OR OUTPUT END OFSILICA ROD.” Input surface 106 may be treated for coupling with outputends 110 of the optical fibers 108. For example, the input surface 106may be polished and/or coated with antireflective coating for free-spacecoupling. The texture of input surface 106 may be prepared to enablefusion splicing with optical fibers 108. In another example, inputsurface 106 may be coated with a surface treatment configured to promotecoupling or fusion of the input surface 106 with the output surfaces 110of the optical fibers 108.

Process 1000 may move to block 1008, “COUPLE A PLURALITY OF OPTICALFIBERS TO THE INPUT SURFACE AREA OF THE TAPERED SILICA ROD.” One or moreoptical fibers 108 may be coupled to input surface 106 by a variety ofmethods, as discussed previously.

FIG. 11 illustrates an example process 1100 for assembling an opticalfiber combiner 400. A hollow structure 210 may enable alignment ofoptical fibers 108 and coupler 102 during fabrication of opticalcombiner 400 and may secure optical fiber 108 and coupler in place aswell as provide optical fibers 108 and/or coupler 102 protection fromenvironmental contaminants or other hazards such as excess heat orpercussion. Process 1100 may begin at block 1102, “DISPOSE HOLLOWSTRUCTURE OVER COUPLER.” At block 1102, hollow structure 210 may beloosely disposed over coupler 102 where an inner diameter 492 of centralaperture 214 is greater than an outer diameter 490 of coupler 102 toenable fitting of hollow structure 210 around coupler 102 withoutsignificantly damaging coupler 102 or hollow structure 210.

Process 1100 may move to block 1104, “THREAD PLURALITY OF OPTICAL FIBERSTHROUGH HOLLOW STRUCTURE.” During assembly, bundle 112 of optical fibers108 may be fed through central aperture 214 of hollow structure 210 foralignment with surface 106. Inner diameter 492 of hollow structure 210at this point in the assembly of combiner 400 is greater than thediameter 494 of bundle 112 of optical fibers 108 to facilitate threadingof optical fibers 108 through hollow structure 210. During assembly,having inner diameter 492 of hollow structure 210 greater than thediameter 494 of bundle 112 of optical fibers 108 may also facilitatealignment of surfaces 110 of optical fibers 108 with surface 106 ofcoupler 102.

Process 1100 proceeds to block 1006, “COLLAPSE HOLLOW STRUCTURE AROUNDOPTICAL FIBERS AND/OR COUPLER.” Hollow structure 210 may be collapsedover coupler and/or optical fibers by any known method such as byapplying heat, mechanical pressure, vacuum suction, or the like or anycombinations thereof. In an example, heat can be applied to hollowstructure 210 to collapse it around coupler 102 and bundle 112. Heat maybe applied by a variety of methods including using a CO₂ laser, plasmaand/or resistive heating, or the like or any combinations thereof.Collapse may be executed before or after surfaces 110 are coupled withsurface 106. In another embodiment, hollow structure 210 may be removedfrom assembly 400 after alignment and/or coupling of optical fibers 108with surface 106 of coupler 102.

Process 1100 may move to block 1108, “COUPLE A PLURALITY OF OPTICALFIBERS TO THE INPUT SURFACE AREA OF THE TAPERED SILICA ROD.” One or moreoptical fibers 108 may be coupled to input surface 106 by a variety ofmethods, as discussed previously.

Although processes 900, 1000 and 1100 have been described as havingseveral steps, it is not necessary for all of the steps of theseprocesses to be performed nor is there a particular order in which thesteps are to be practiced within the scope of the contemplated subjectmatter. Although processes 900, 1000, and 1100 above are described inthe context of fabrication of combiner assembly 400, such description isfor the sake of simplicity and is not intended to be limiting in anymanner. Processes 900, 1000, and 1100 may be applied in fabrication ofany example embodiments described, suggested or contemplated herein.Furthermore, unidentified intervening steps may be contemplated andpracticed within the scope of the presently disclosed technology.

FIG. 12A-12D depict examples of hollow structures having differingsymmetries. FIG. 12E depicts graphs of beam-parameter product (BPP)conversion efficiency for various hollow structure outer-surfacesymmetries. BPP conversion efficiency improves with increased asymmetrydue in part to the fact that there is enhanced mode-scrambling orskew-rays bouncing over the length of the coupler in a non-circularouter-surface geometry. FIG. 12A is a sectional view of a bundle 112 ofoptical fibers 108 disposed in a symmetric hollow structure 210. Asdescribed above, hollow structure 210 may encase optical fibers 108 andfacilitate coupling of optical fibers 108 to coupler 102. FIG. 12Edepicts graphs of beam-parameter product (BPP) conversion efficiency forvarious hollow structure symmetries. Graph 1208 corresponding to acoupler assembly including hollow structure 210 depicted in FIG. 12Ashows the BPP conversion efficiency to be ˜90%.

FIG. 12B is a sectional view of a bundle 112 of optical fibers 108disposed in an asymmetric hollow structure 1202. Hollow structure 1202has one symmetry breaking feature 1204. Referring now to FIG. 12E, graph1210 corresponding to a coupler including hollow structure 1202 in FIG.12B shows the related BPP conversion efficiency to be ˜94-95%.

FIG. 12C is a sectional view of a bundle 112 of optical fibers 108disposed in an asymmetric hollow structure 1216. Hollow structure 1216has two symmetry breaking features 1206 and 1220. Referring now to FIG.12E, graph 1212 corresponding to a coupler including hollow structure1216 in FIG. 12C shows the related BPP conversion efficiency to be˜95-97%.

FIG. 12D is a sectional view of a bundle 112 of optical fibers 108disposed in an asymmetric hollow structure 1218. Hollow structure 1218has three symmetry breaking features 1222, 1224 and 1226. Referring nowto FIG. 12E, graph 1214 corresponding to a coupler including hollowstructure 1218 in FIG. 12D shows the related BPP conversion efficiencyto be ˜95-97%.

Having described and illustrated the general and specific principles ofexamples of the presently disclosed technology, it should be apparentthat the examples may be modified in arrangement and detail withoutdeparting from such principles. We claim all modifications and variationcoming within the spirit and scope of the following claims.

The invention claimed is:
 1. A method for fabricating an optical fibercombiner comprising: exposing an output end of a silica rod to anetchant; gradually exposing a length of the silica rod lengthwise to theetchant over a period of time; ending the etching at an input end of thesilica rod to etch a gradual taper into the length of the silica rod bygradually exposing the length of the silica rod to the etchant over theperiod of time, wherein the etching begins by exposing the output end tothe etchant first and then gradually exposing the entire length of thesilica rod ending the etching with the input end causing the input endto have a greater surface area A_(in) than the surface area A_(out) ofthe output end; collapsing a hollow structure around a plurality ofoptical fibers and at least a portion of the length of the silica rodproximate the input end so as to secure the plurality of optical fibersin place; and coupling the plurality of optical fibers to the input end.2. The method of claim 1, wherein the optical fibers include a claddingstructure and wherein the hollow structure has a higher index ofrefraction compared to an index of refraction of the claddingstructures.
 3. The method of claim 1, further comprising graduallyexposing the silica rod to the etchant at a constant rate for a fixedperiod of time wherein the fixed period of time is determined by anoutput diameter of the silica rod.
 4. The method of claim 1, furthercomprising polishing the input end of the silica rod.
 5. The method ofclaim 1, wherein the etching is wet etching.
 6. The method of claim 1,further comprising coating the input surface with an antireflectivecoating for free-space coupling.
 7. The method of claim 1, furthercomprising coating the input surface with a surface treatment configuredto enable fusion splicing of the input fibers.
 8. A method forfabricating an optical fiber combiner comprising: disposing a first endof a hollow structure around an input surface of a coupler, the couplercomprising a silica rod having a tapered outer surface and an outputwaveguide, wherein the coupler has a higher index of refraction comparedto the hollow structure; threading a plurality of optical fibers from asecond end of the hollow structure through a length of an inner apertureof the hollow structure; coupling the plurality of optical fibers withthe input surface of the coupler; and collapsing the hollow structureonto the coupler and the plurality of optical fibers.